1. Field of the Invention
The present invention relates to a light irradiation apparatus, a light irradiation method, a crystallization apparatus, a crystallization method, a device, and a light modulation element.
2. Description of the Related Art
A thin film transistor (TFT) which is used for a switching element or the like which selects a display pixel in, e.g., a liquid crystal display (LCD) has been conventionally formed in an amorphous silicon layer or a polysilicon layer.
The polysilicon layer has a higher mobility of electrons or holes than the amorphous silicon layer. Therefore, when a transistor is formed in a polysilicon layer, a switching speed is increased and a display response speed is thus increased as compared with a case where a transistor is formed in an amorphous silicon layer. Further, a peripheral LSI can be thereby formed of a thin film transistor. Furthermore, there is an advantage that a design margin of any other component can be reduced. Moreover, peripheral circuits such as a driver circuit or a DAC can be operated at a higher speed when these peripheral circuits are incorporated in a display.
Since polysilicon is formed of an aggregation of crystal grains, when, e.g., a TFT transistor is formed, a crystal grain boundary or boundaries exist in a channel area of such a transistor, and the crystal grain boundaries become an obstacle and lower the mobility of electrons or holes as compared with single-crystal silicon. Additionally, in case of many thin film transistors formed to polysilicon, the number of crystal grain boundaries formed in channel portions differs between the respective thin film transistors, and this becomes unevenness in transistor characteristics and leads to a problem of display irregularities in case of a liquid crystal display. Thus, in recent years, in order to improve the mobility of electrons and holes and reduce irregularities in the number of crystal grain boundaries in channel portions, there has been proposed a crystallization method which generates crystallized silicon having crystal grains with a particle size which is as large as one channel area can be formed in each crystal grain.
As this type of crystallization method, there has been conventionally known a “phase control ELA (Excimer Laser Annealing) method” which generates a crystallized semiconductor film by irradiating, with an excimer laser light, a phase shifter which is closely arranged in parallel to a non-single-crystal semiconductor film such as a polysilicon semiconductor film or an amorphous semiconductor film. The detail of the phase control ELA method is described in, e.g., Journal of the Surface Science Society of Japan Vol. 21, No. 5, pp. 278-287, 2000.
In the phase control ELA method, a light intensity distribution having an inverse peak pattern (a pattern in which a light intensity is minimum at the center (a minimum light intensity portion) and the light intensity is suddenly increased toward the periphery or in the lateral direction) in which the light intensity is lower than that at the periphery is generated at a point or a line corresponding to a phase shift portion of a phase shifter, and a non-single-crystal semiconductor film is irradiated with a light having this inverse-peak-shaped light intensity distribution so that the non-single-crystal semiconductor film is fused. As a result, a temperature gradient is generated in a fusing area according to the light intensity distribution in an irradiation target area, a crystal nucleus is formed at a part which is solidified first or not solidified in accordance with the minimum intensity portion, and a crystal grows in a lateral direction from this crystal nucleus toward the periphery (which will be referred to as a “lateral growth” or a “growth in a lateral direction” hereinafter), thereby generating a crystal grain with a large particle size.
Further, “Growth of Large Si Grains at Room Temperature by Phase-Modulated Excimer-Laser Annealing Method” by H. Ogawa et al., IDW'03 Proceedings of The 10th International Display Workshops, p. 323 brings forth a crystallization method which generates a crystal grain by irradiating a non-single-crystal semiconductor film with a light having a V-shaped light intensity distribution obtained through a phase shifter and an image formation optical system. This reference discloses that it is desirable for an intensity distribution of a light which irradiates the non-single-crystal semiconductor film to vary in a V shape in a range of 0.5 to 1.0 when its maximum value is standardized as 1.0. The “light intensity distribution with the inverse peak pattern” and the “V-shaped light intensity distribution” have the same function as seen from each central portion (the minimum light intensity distribution and the vicinity thereof). The both light intensity distribution are written as a “V-shaped light intensity distribution” in the present invention.
The inventors of the present application proposes a light modulation element which can obtain a V-shaped light intensity distribution by a combination with an image formation optical system in, e.g., Jpn. Pat. Appln. No. 2003-117486 (which will be referred to as a “related application” hereinafter). The light modulation element proposed in the related application is a “binary modulation type” phase shifter having a reference phase value of 0 degree and a modulation phase value of 90 degrees, i.e., two types of modulation phase values, which has a phase modulation whose dimension is not greater than a point spread function range of an image formation optical system when converted into a counterpart on a light modulation element. Incidentally, it is needless to say that disclosed matters in the related application do no constitute a prior art of the present application.
Specifically, as shown in FIG. 14, a typical light modulation element proposed in the related application has a reference phase area (indicated by a blank portion in the figure) 10a having a reference phase value of 0 degree and rectangular phase-modulation areas (indicated by shaded portions in the figure) 10b having a modulation phase value of 90 degrees. An occupied area ratio (a duty) of the phase-modulation areas 10b to the reference phase area 10a linearly varies between 0% and 50% along a horizontal direction (a lateral direction) (along an X cross section) in the figure. Concretely, an occupied area ratio of the phase-modulation areas 10b is 0% on both sides of a phase pattern along the horizontal direction, and an occupied area ratio of the phase-modulation areas is 50% at the center.
When such a light modulation element and an image formation optical system which forms an image of a light modulated by this light modulation element on a non-single-crystal semiconductor film are used, such a V-shaped light intensity distribution as shown in FIGS. 15A to 15C are obtained on the non-single-crystal semiconductor film. The light intensity distributions shown in these figures are calculated on the assumption that a wavelength λ of an incident light beam is 248 nm, an image side numerical aperture NA of the image formation optical system is 0.13 and a value σ (a coherence factor) of the image formation optical system is 0.47. A light intensity I obtained at a focus position or plane of the image formation optical system is dependent on an occupied area ratio D (which varies between 0 and 0.5 in the example shown in FIG. 14) of the phase-modulation areas 10b at a position in question along the X cross section, and approximately represented by the following expression.I≈(4−4A)|D−0.5|2+A 
where A≈cos2(θ/2)
In this expression, θ is a modulation phase value (90 degrees in the example shown in FIG. 14), and A is a standardized value of a light intensity obtained at a focus surface position corresponding to a position where an occupied area ratio D is 50% on the maximum level (a value when a maximum value of a light intensity in a V-shaped light intensity distribution is standardized as 1.0).
When the light modulation element shown in FIG. 14 is used, a substantially ideal V-shaped light intensity distribution which is substantially symmetrical in a lateral direction with a minimum intensity portion at the center and whose formation position is controlled is formed at a focus position (an image formation surface) of the image formation optical system as shown in FIG. 15B. However, not only a shape of the V-shaped light intensity distribution to be formed changes but is asymmetrically varies depending on a defocusing direction at a position slightly moved from the focus position by 10 μm in a direction closer to the image formation optical system as shown in FIG. 15A (a defocus position of −10 μm) or at a position slightly moved from the focus position by 10 μm in a direction apart from the image formation optical system as shown in FIG. 15C (a defocus position of +10 μm).
A board thickness deviation which can be a factor of defocusing unavoidably exists in a glass sheet used as a processed substrate having a non-single-crystal semiconductor film formed thereon, which should be irradiated with a light. As a result, a shape of the V-shaped light intensity distribution asymmetrically changes due to defocusing, and a desired V-shaped light intensity distribution cannot be stably formed on a non-single-crystal semiconductor film. Therefore, there occurs an inconvenience that crystal grains cannot be generated in substantially equal sizes in a semiconductor film on the processed substrate. Specifically, when slightly moved from the focus position by −10 μm as shown in FIG. 15A, a light intensity distribution in which two minimum light intensity portions exist is formed, and hence a crystal growth start point is divided into two points, thereby reducing a size of the proper crystal grain. Furthermore, as shown in FIG. 15C, in a light intensity distribution formed at a position slightly moved from the focus position by +10 μm, it can be understood that an uncrystallized area expands and a “filling factor of crystal grains” is reduced. Here, “the filling factor of crystal grains” is a ratio of a crystallized area to a light irradiation area when a non-single-crystal semiconductor film is irradiated with a light beam having a V-shaped light intensity distribution.
In a binary modulation type light modulation element, when a modulation phase value is set to 180 degrees rather than 90 degrees mentioned above, a V-shaped light intensity distribution to be formed does not asymmetrically change in dependence on the defocusing direction. However, an attempt to assure the minimum light intensity portion to be large to some extent in the V-shaped light intensity distribution with a modulation phase value being set to 180 degrees restricts a distribution of an occupied area ratio of phase-modulation areas having the modulation phase value to 0% to around 100%. This means that a phase area having the modulation phase value or the reference phase value becomes extremely small, and also means that production of the binary modulation type light modulation element whose modulation phase value is 180 degrees is practically difficult.